1. Field of the Invention
This invention relates to operational amplifiers, and more specifically to a CMOS operational amplifier having good performance rail-to-rail.
2. Description of the Prior Art
This invention relates to rail-to-rail input/output general purpose operational amplifiers ("op amps"). Such amplifiers typically are used widely in the electronics field. It is desirable to have an input stage to such an operational amplifier with full "rail-to-rail" operational range. Rail-to-rail refers to the range from the positive supply voltage (designated VDD) to the negative voltage supply (designated VSS) which is often ground, and in this context refers to the common node voltage of the input signal to the input stage being anywhere within this range.
In the prior art, the input stage of such an operational amplifier is typically as shown in FIG. 1, including a differential pair of P-channel transistors Q2, Q4 and a differential pair of corresponding N-channel transistors Q3, Q5. Typically this is a CMOS integrated circuit hence these are respectively PMOS and NMOS transistors. As can be seen, the positive input signal applied to terminal VINP is coupled to the gates of transistors Q4 and Q5, while the minus input signal applied to terminal VINM is coupled to the gates of transistors Q2 and Q3. As is well known in the application of operational amplifiers in a negative feedback system, the input signals which are applied to input terminals VINP, VINM in FIG. 1 have the same DC voltage component and differ in their AC component. The level of DC voltage of the two input signals is usually called the "common mode" or "common-mode voltage".
There are three basic operating regions of such an input stage. When the common mode voltage is near the negative power supply voltage (the voltage applied to the negative power supply terminal or "rail" VSS), only the P-channel pair of transistors Q2 and Q4 operates, i.e. is conductive. For a common mode voltage near the positive power supply voltage (the voltage applied to the positive power supply terminal or "rail" VDD), only the N-channel pair of transistors Q3 and Q5 operates. For a common mode voltage which is so-called "mid rail", i.e. halfway between the VDD and VSS voltages, both differential pairs Q2, Q4 and Q3, Q5 operate. Therefore the transconductance (g.sub.m) can change by a factor of two as the common mode voltage moves from the level of the one power supply voltage to the level of the other i.e., rail-to-rail. (Transconductance is defined as the relationship between current and gate-source voltage for a transistor expressed e.g. as milliamps/volt). This change in transconductance complicates the operational amplifier frequency compensation and other parameters, and hence is recognized as being undesirable.
As seen in FIG. 1, the output signals from this input stage are provided on lines VOUT1, VOUT2, VOUT3 and VOUT4. Also in FIG. 1, control signals P1 and N1 respectively control transistors Q10, Q14, which are current sources supplied by, respectively, the positive and negative voltage supplies.
Prior art operational amplifiers using a circuit like that of FIG. 1, where it is desired to have rail-to-rail functionality, typically use a MOS transistor translinear control circuit to regulate the "tail current" to achieve constant transconductance, or use matching N-channel transistors with P-channel transistors, or a current monitor circuit, to control the transconductance. (Tail current is the current supplied by transistors Q10, Q14.) All these techniques produce somewhat constant transconductance in operation. As the input signal swings from rail-to-rail, the currents supplied by the positive and negative rails change because they are controlled by the translinear circuit. As the currents change, the amount of power drained from the positive and negative rails changes, which in turn is injected as an AC signal into the rails. In other words, as the common mode voltage moves from near the voltage of the positive power supply rail to near the voltage of the negative power supply rail, or vice versa, the supply current is changed in order to keep transconductance constant. However this injects a small AC signal into the power supply lines ("rails") (connected to terminals VDD, VSS) because of the changing supply current. This small AC signal in the power supply rails not only degrades their own power supply rejection ratio (PSRR) but also degrades performance of any other circuits connected to the same power supply rails. Hence the prior art solutions are not adequate.